The invention relates generally to a method and apparatus for setting a cement plug in the wide-mouth shaft of an earth cavern. During setting of the cement plug, it is initially supported by a ballon-type structure, which is inflated with a liquid or gas after lowering it into the cavern.
It is common practice to store gases and liquids, for later use, in underground caverns. This technique does not involve the time and expense required to construct storage equipment above ground, and it is a safer mode of storage for many liquids and gases. As an example of underground storage, empty salt caverns are sometimes used for temporary storage of hydrocarbons. After salt is removed from a salt dome, there is a production casing which extends from the earth's surface into the mouth of the cavern. This casing is surrounded by a layer of cement which bonds the casing to the original borehole.
Before the hydrocarbon (usually crude oil, or a petroleum product or chemical) is pumped into a salt cavern, the cavern is filled with brine, through a brine injection pipe which hangs inside the production casing and extends below the casing to a point near the cavern bottom. The hydrocarbon is then pumped into the cavern through the annulus (space) between the production casing and the brine injection pipe. Since the hydrocarbon has a lower density than the brine, it floats on the surface of the brine as a separate phase. When it is desired to withdraw the hydrocarbon for use, more brine is pumped down the injection pipe to raise the brine level in the cavern. As the brine level rises, it displaces an equivalent volume of the hydrocarbon, which is carried up to the surface through the casing annulus.
In some salt caverns, the removal of liquids, such as hydrocarbons, causes a substantial part of the casing cement layer and the salt formation next to the cement layer to be eroded away. The erosion is caused partly by turbulence created in withdrawing the hydrocarbon from the cavern, and partly from using brine which is not salt saturated to displace the hydrocarbon. Saturated brine is a solution containing about 37 percent by weight salt. When a less concentrated brine is used, the salt defining the cavern walls tends to migrate into the brine solution to satisfy the saturated condition. As the erosion progresses, a substantial part of the lower end of the production casing is left hanging in the cavern in an unsupported condition. The unsupported condition is remedied by periodically cutting off the lower end of the casing to shorten it. If the erosion is not halted, it can eventually reach a point where there is danger of the hydrocarbon polluting the potable water aquifers in the rock strata above the cavern.
It is believed that the erosion problem can be overcome by sealing off part of the mouth of the salt cavern below the end of the existing production casing with a solid plug of cement. A hole is then drilled through the cement plug into the cavern and a new production casing is cemented into the hole to provide means for storing and retrieving liquids or gases from the cavern. Heretofore, it has not been possible to perform the type of cementing job which is required. The known tools used in downhole cementing operations, which include inflatable packers, are not designed for supporting the cement plug in the wide-mouth shaft of an earth cavern.